Adsorption

The term adsorption was first coined in 1881 by a German physicist named Heinrich Kayser. Adsorption is often described as a surface phenomenon where particles are attached to the top layer of material. It normally involves the molecules, atoms or even ions of a gas, liquid or solid in a dissolved state that is attached to the surface.

Adsorption is mainly a consequence of surface energy. Generally, the surface particles which can be exposed partially tend to attract other particles to their site. Interestingly, adsorption is present in many physical, natural, biological and chemical systems and finds its use in many industrial applications. We will learn about the concept in detail below.

Table of Contents

• What Is Adsorption?
• Types of Adsorption
• Physisorption and Chemisorption Adsorption Characteristics
• Adsorption Isotherm
• Differences between Absorption and Adsorption
• Applications of Adsorption
• Adsorption Questions

What Is Adsorption?

Adsorption is a process that involves the accumulation of a substance in molecular species in higher concentrations on the surface. If we look at Hydrogen, Nitrogen and Oxygen, these gases adsorb on activated charcoal. Also, we have to note that adsorption is different from absorption. The two processes involve totally different mechanisms.

For the adsorption process, two components are required:

• Adsorbate: Substance that is deposited on the surface of another substance. For example, H₂, N₂ and O₂ gases.
• Adsorbent: Surface of a substance on which adsorbate adsorbs. For example, Charcoal, Silica gel, and Alumina.

Also Read: Surface Chemistry

Adsorption Enthalpy

Adsorption constants are basically equilibrium constants. What it means is that they also follow or obey the Van ‘t Hoff equation:

In this formula, the variation of K must be isosteric, that is, at constant coverage.

Starting from BET isotherm and assuming that the entropy change is the same for liquefaction and adsorption, we obtain

Precisely, we can say that adsorption is more exothermic than liquefaction.

Types of Adsorption

On the basis of interaction forces between adsorbate and adsorbent, adsorption is of two types. 

1. Physical adsorption

This type of adsorption is also known as physisorption. It is due to weak Van der Waals forces between adsorbate and adsorbent. 

For example, H₂ and N₂ gases adsorb on coconut charcoal.

2. Chemical adsorption

This type of adsorption is also known as chemisorption. It is due to strong chemical forces of bonding type between adsorbate and adsorbent. We can take the example involving the formation of iron nitride on the surface when the iron is heated in N₂ gas at 623 K. 

The adsorption of a gas on a solid is a spontaneous exothermic reaction. The amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.

Physisorption and Chemisorption Adsorption Characteristics

Characteristics of physical adsorption:

1. This type of adsorption is caused by physical forces.
2. Physisorption is a weak phenomenon.
3. This adsorption is a multi-layered process.
4. Physical adsorption is not specific and takes place all over the adsorbent.
5. Surface area, temperature, pressure, and nature of adsorbate effects physisorption.
6. Energy for activation is low (20 – 40 kg/mol).

Characteristics of chemical adsorption:

1. This type of adsorption is caused by chemical forces.
2. It is a very strong process.
3. This type of adsorption is almost a single-layered phenomenon.
4. Chemisorption is highly specific and takes place at reaction centres on the adsorbent.
5. Surface area, temperature, and nature of adsorbate effects chemisorption.
6. The energy of activation is very high, 40 – 400 kJ/mol.

Adsorption Isotherm

Adsorption is usually described by isotherms. It is due to the fact that temperature plays an important role or that it has a great effect on the whole process. Moreover, there are several isotherm models that are used to describe the adsorption technique. These include the following:

Freundlich Theory

Freundlich adsorption isotherm is obeyed by the adsorption, where the adsorbate forms a monomolecular layer on the surface of the adsorbent.

x represents the amount of gas adsorbed on the m gram of adsorbent, K and n are adsorption constants, and ‘p’ is pressure n always greater than one. 

A major drawback of Freundlich adsorption isotherm is that it fails at high pressure. It could not explain the multi-layered adsorption process.

Langmuir Theory

In 1916, Langmuir proposed the theory of adsorption of a gas on the surface of the solid to be made up of elementary sites, each of which would adsorb one gas. It is assumed that all adsorption sites are equivalent, and the ability of a gas molecule to get bound to any one site is independent of whether or not the neighbouring sites are occupied. Additionally, it is also assumed that dynamic equilibrium exists between adsorbed and non-adsorbed gas molecules.

The following principles can be obtained from Langmuir adsorption isotherm: 

• The gas adsorbed behaves ideally in a vapour phase.
• Only monolayer adsorption takes place.
• The surface of the solid is homogeneous.
• There is no lateral interactive force between the adsorbate molecule.
• The adsorbed gas molecules are localised.

BET theory (after Brunauer, Emmett, and Teller)

The BET theory was proposed by Brunauer, Emmett and Teller in the year 1938. This theory explains the formation of multilayer adsorption during physisorption. This theory also talks about the uniformity in the sites of adsorption of solid surfaces. It assumes that when adsorption occurs at one site, it will not affect adsorption at neighbouring sites.

Applications of Adsorption

1) Air pollution masks

These consist of silica gel or activated charcoal powder; when dust or smoke are paused through them, these particles get adsorbed on the surface of these materials.

2) Separation of noble gases by Dewar’s flask process

A mixture of noble gases of Ne, Ar, and Kr is passed through Dewar’s flask in the presence of heated coconut charcoal. Argon and Krypton gels adsorbed, leaving Neon.

3) Purification of water

By the addition of alum stone to the water, impurities get adsorbed on the alum, and the water gets purified.

4) Removal of moisture and humidity

Moisture in the air is removed by placing silica gel on which water molecules gets adsorbed.

5) Adsorption chromatography

It is used to separate pigments and hormones.

6) Ion exchange method

In this method of removing the hardness of water, calcium and magnesium ions get adsorbed on the surface of the ion exchange resin.

7) In metallurgy

In the froth floatation process of concentration of ore, the particle gets adsorbed on the froth.

Differences between Absorption and Adsorption

Different Instances of Adsorption

Water Adsorption

If we talk about chemical engineering, materials science, and even catalysis, the adsorption of water at surfaces is of great importance in these fields. Adsorption of water, also known as surface hydration, is where the physically or chemically adsorbed water is present on the surfaces of solids. This plays an essential role in governing interface properties, chemical reaction pathways and catalytic performance in a wide range of systems.

In the case of physically adsorbed water, surface hydration is generally eliminated by the drying process that occurs at conditions of temperature and pressure which further leads to the full vaporization of water. As for chemically adsorbed water, hydration may be in the form of either dissociative adsorption or molecular adsorption.

Adsorption in Viruses

If you understand the virus cycle, the first step is adsorption, followed by penetration, uncoating, synthesis and release. Usually, the virus replication cycle is similar for different types of viruses.

Polymer Adsorption

Adsorption of molecules is found on the surfaces of the polymer. This characteristic is important in a number of use-case applications. For example, it is important in the development of non-stick coatings and in several biomedical devices. Using the process of polyelectrolyte adsorption, polymers can also be adsorbed to surfaces.

Adsorption on catalysts

When adsorption occurs on molecules on certain catalytic materials, there is usually an acceleration of certain chemical reactions.

Adsorption Questions

1. Adsorption of gas on a solid is always exothermic. Explain.

Solution:

The adsorption of a gas on a solid is a spontaneous process. When a gas adsorbs on the solid due to the molecular interaction process, the entropy of gas molecules decreases. To make the process spontaneous, adsorption must be exothermic.

2. Among SO₂, CH₄, and H₂ gases which gas adsorbs more on the charcoal and why?

Solution:

Sulphur dioxide adsorbs more than methane and hydrogen since the critical temperature of sulphur dioxide is higher than methane and hydrogen.

3. What do you mean by critical temperature? What is the relation between critical temperature and the gas adsorbed?

Solution:

The temperature above which a gas cannot be liquefied even by the application of high pressure is known as critical temperature. Greater is the critical temperature (To) more in the gas adsorbed on the surface.

4. What are the factors for the effective adsorption of gases on solids?

Solution:

1. Nature of gases: Ease is liquefication more is gas adsorbed.
2. The surface area of adsorbent: The more the surface area, more gas is adsorbed.
3. Temperature: Physisorption increases with a decrease in temperature, while chemisorption increases with an increase in temperature.
4. Pressure: It affects only physisorption; if pressure increases, the amount of gas adsorbed also increases.

5. Define activation.

Solution:

When adsorbents are heated in a vacuum from 573-623 K, the surface area is increased, and it is known as activation.

6. Write about adsorption from solutions.

Solution:

Charcoal pieces are added to the acetic acid solution. Some of the acetic acid molecules adsorb on charcoal. This depends upon the following:

1. Nature of adsorbate
2. The surface area of the adsorbent 
3. Temperature 
4. Concentration